In your 'Opponent Manager' do you happen to have 'Display Active Drivers Only' checked/selected? If so, uncheck it and any new drivers should be at the bottom of the list. Select whoever you want and be sure to 'Save' the roster.
As for the 'Weekend' problem...is that in Testing, Single, Career? I'm not sure that I've ever seen a 'Weekend' option in the game.
Also, I realize that problems within the game can, at times, be extremely frustrating, but a single sentence describing your problem isn't very much to go by. The more info you supply (what you did, how you did what you did, etc.) would go a long way towards your getting help. You don't go to a mechanic and say, 'my car makes a funny noise' and expect them to know exactly what the problem is. OK...some people do do that.